Geologic formations are used for many purposes such as hydrocarbon production, geothermal production and carbon dioxide sequestration. In general, formations are characterized in order to determine whether the formations are suitable for their intended purpose.
One way to characterize a formation is to convey a downhole tool through a borehole penetrating the formation. The tool is configured to perform measurements of one or more properties of the formation at various depths in the borehole to create a measurement log.
Many types of logs can be used to characterize a formation. One type of downhole tool that can determine various properties of a formation is a nuclear magnetic resonance (NMR) tool. NMR tools may generate a static magnetic field in a sensitive volume surrounding the wellbore or may use the earth's magnetic field rather than generating a magnetic field. NMR is based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves in part along an externally applied magnetic field, resulting in an equilibrium macroscopic nuclear magnetization. This equilibrium situation can be disturbed by a pulse of a magnetic field oscillating at the Larmor frequency, which tips the magnetization within the bandwidth of the oscillating magnetic field away from the static field direction.
After tipping, the magnetization precesses around the static field at a particular frequency known as the Larmor frequency. At the same time, the magnetization returns to the equilibrium direction (i.e., aligned with the static field) according to a characteristic relaxation time known as the spin-lattice relaxation time or T1.
At the end of a θ=90° tipping pulse (also referred to as an excitation pulse), the magnetization points in a common direction perpendicular to the static field and then precesses at the Larmor frequency. However, because of inhomogeneity in the static field due to the constraints on tool shape, imperfect instrumentation, or microscopic material heterogeneities, each nuclear spin precesses at a slightly different rate. Hence, after a time long compared to the precession period, but shorter than T1, the spins will no longer be precessing in phase. This de-phasing occurs with a time constant that is commonly referred to as T2*. In downhole applications, T2* is mainly due to the non-uniformity of the static magnetic field. T2* is often so short that the NMR signal that forms right after the tipping pulse is undetectable. It is, however, possible to rephase the spins by using so-called rephasing or refocusing pulses to generate a sequence of spin echoes. The standard pulse echo sequence for doing this is the Carr-Purcell-Meiboom-Gill (CPMG) sequence. The decay of the amplitudes of the spin echoes occurs with the spin-spin relaxation time T2 and is due to properties of the material. Hence, a CPMG consists of one excitation pulse followed by a plurality of refocusing pulses, with the decaying NMR echoes forming between the refocusing pulses.
The NMR tool includes a receiving coil designed so that a voltage is induced by the precessing spins. Only that component of the nuclear magnetization that is precessing in the plane perpendicular to the static field is sensed by the coil. Signals received by the receiving coil are referred to as NMR signals and these signals are used to determine properties of the formation in the sensitive volume. NMR signals at the present time are used to determine porosity, hydrocarbon saturation, and permeability of rock formations. It would be well received in the drilling industry if additional properties could be determined using NMR tools.